
n = 51
dp = [[0 for i in xrange(n)] for j in xrange(n)]
dp[0][0] = 1

for i in xrange(1, n):
	dp[i][0] = dp[i-1][0] + dp[i-1][1]
	for b in xrange(3, i + 1):
		dp[i][1] += dp[i - b][0]

ans = dp[n-1][0] + dp[n-1][1]
print ans 
